Not necessarily. Per ACA §1513(c)(2)(E) you arrive at the number of full time employees by adding the number of full time employees to (the number of hours worked by all other employees divided by 120).
Let's say that a Denny's, Applebee's, or other restaurant always has 2 full-time employees on staff, a manager and assistant manager, and all other employees are not full-time. The threshold is now 33 FTEs (since we have 2 full-timers) and under the ACA calculation those employees need to collectively work 3,960 hours per month. At 30 days per month that's 132 hours per day.
What time does Denny's open, 6? What time do they close, 11? That's a 17 hours business day. At 132 worker hours per business day, they'd need an average staff of 7.75 people to hit that threshold.
What's a non-busy/non-slow staff loadout for a place like Denny's? 3-4 servers, 3-4 cooks, a busboy, a dishwasher, and a shift leader? That's 9-11 employees at "normal" load.
My assumptions may be off, they are assumptions after all, but they go to illustrate that the math is very possible.